The invention relates to a method for adjusting the temperature of an exhaust gas aftertreatment device connected to an internal combustion engine.
DE 10 2005 013 707 A1 discloses a method for adjusting the temperature of an exhaust gas aftertreatment device connected to an internal combustion engine, having an electric heating element, an oxidation catalytic converter connected behind the heating element, and a selective catalytic reduction (SCR) catalytic converter connected behind the oxidation catalytic converter, wherein, after the internal combustion engine is started, current is fed to the heating element, after which an enrichment of the exhaust gas with combustible components is performed upstream of the oxidation catalytic converter. The enrichment of the exhaust gas with combustible components can be performed in this case by a corresponding adjustment of injection parameters of fuel injection processes, particularly if the oxidation catalytic converter has exceeded a predetermined temperature threshold. Likewise, the heating power of the heating element can be adjusted according to temperatures of the catalysts. In this way, it is possible to rapidly heat the catalysts to their operating temperature—for example when the internal combustion engine is started from cold.
The problem addressed by the invention is that of providing a further-improved method for adjusting the temperature of a corresponding exhaust gas aftertreatment device.
This problem is addressed by a method of the present invention.
In the method, temperature values for a current temperature of the oxidation catalytic converter and the SCR catalytic converter are determined continuously, and depending on the determined temperature values, injection parameters of injection processes for fuel injections into the combustion chambers of the internal combustion engine, and a heat output of the heating element, are adjusted. Here the oxidation catalytic converter and the SCR catalytic converter are each assigned a characteristic temperature value which is linked to a predetermined conversion threshold. According to the invention, a first characteristic temperature value for an oxidative carbon monoxide conversion, and a second characteristic temperature value for an oxidative hydrocarbon conversion, are assigned to the oxidative catalytic converter, and a third characteristic temperature value for a reductive nitrogen oxide conversion is assigned to the SCR catalytic converter, wherein when the first and second characteristic temperature values for the temperature of the oxidation catalytic converter are reached, and when the third characteristic temperature value for the temperature of the SCR are reached, in each case different values for the injection parameters and/or the heating power of the heating element are adjusted. This approach takes into account the fact that a certain degree of the catalytic activity of the catalysts for different types of catalyzed reactions is achieved at different temperatures. Therefore, the approach according to the invention enables a particularly effective thermal management of the exhaust gas aftertreatment device by considering these reactions. The catalysts can be brought from low temperatures, on the one hand, particularly quickly and efficiently to their respective operating temperatures, and on the other hand in adverse operating conditions can be particularly efficiently protected from cooling down to below their respective operating temperatures.
By setting injection parameters for the fuel injection according to the corresponding characteristic temperature values for the carbon monoxide and hydrocarbon conversion rate of the oxidation catalytic converter, which particularly varies at low temperatures, it is then possible to produce fractions of carbon monoxide (CO) and hydrocarbon (HC) in the exhaust gas which are optimized as a result of their oxidation for effective heat release. The heat energy of the heating element can then, on this basis, be adjusted to thereby enable an efficiency-optimized division of the heat energy into the oxidation catalytic converter and the heat energy released by the heating element. Moreover, an optionally modified setting of the injection parameters, additionally according to the third characteristic temperature value for the temperature of the SCR catalytic converter, for a predetermined conversion threshold of, for example, 50%, makes it possible for the SCR catalytic converter to quickly reach its level of catalytic efficacy with respect to a reductive conversion of nitrogen oxides (NOx) while preventing a drop below this temperature value.
In one embodiment of the method, a lower value is used for the first characteristic temperature value for an oxidative hydrocarbon conversion of the oxidation catalytic converter than for the second characteristic temperature value for oxidative carbon monoxide conversion thereof. This approach takes into account the fact that the oxidation catalytic converter typically reaches a predetermined value of, for example, 50% for a catalytic oxidative conversion of carbon monoxide at lower temperatures than for the same degree of catalytic oxidative conversion of hydrocarbon.
In a further embodiment of the method, higher values are used for the first characteristic temperature value and/or for the second characteristic temperature value and/or the third characteristic temperature value for increasing temperatures than are used for decreasing temperatures. The inventors have surprisingly found that, with respect to a particular conversion rate of, for example, 50%, a temperature hysteresis occurs such that as the temperature increases, a jump in the catalytic activity (so-called light-off) occurs at a higher temperature of the respective catalyst, and an extinction of the catalytic activity occurs with decreasing temperature (so-called light-out). By taking this phenomenon into account in the measures provided for a thermal management operation of the heating element and/or a modification of the injection parameters, the invention therefore enables a more efficient use of energy.
In a further embodiment of the method, a state of aging of the oxidation catalytic converter and/or the SCR catalytic converter is/are determined and the first and/or second and/or third characteristic temperature values are changed according to the determined state of aging in each case. In this way, age-related deterioration of the catalytic activity of the catalysts can be taken into account, particularly in the operation of the electric heating element, and largely compensated for. The respective aging states can be determined, by way of example, from time to time by determining current conversion values and comparing these to stored comparative conversion values. In addition, the thermal load of the catalysts can be detected, and this can be incorporated into an aging factor.
In a further embodiment of the method, for the heating of the oxidation catalytic converter and/or the SCR catalytic converter, a first heat energy is released as a result of operation of the heating element and, according to the set injection parameters, a second heat energy is released as a result of a late timing of fuel burnt in the combustion chambers of the internal combustion engine, and a third heat energy is released at the oxidation catalytic converter as a result of a post-combustion of incompletely combusted fuel discharged by the internal combustion engine, wherein a relative fraction of each heat energy in the total heat energy resulting from the released heat energy is adjusted as a function of the determined temperature values and the first characteristic temperature value and/or the second characteristic temperature value and/or the third characteristic temperature value. This enables a particularly efficient thermal management of the catalysts.
It has been found that this is particularly the case if, in a further embodiment of the invention, the total heat energy is only applied by the first heat energy and the second heat energy when, for the current temperature of the oxidation catalytic converter, temperature values are determined to be below the first characteristic temperature value. Furthermore, it has been found to be particularly effective if the relative fraction of the first heat energy is adjusted to about 50% in cases where, for the current temperature of the oxidation catalytic converter, the temperature values are determined to be below the first characteristic temperature value.
Furthermore, it is particularly advantageous if, in a further embodiment of the invention, the relative fraction of the first heat energy is adjusted to less than about 30% in cases where current temperature values for the temperature of the oxidation catalytic converter are determined to be above the first characteristic temperature value, and the temperature values for the current temperature of the SCR catalytic converter are determined to be below the third characteristic temperature value.
Finally, it has proven to be particularly advantageous, in a further embodiment of the invention, that if temperature values are determined to be below the first characteristic temperature value for the current temperature of the oxidation catalytic converter, the total heat energy is adjusted lower than if temperature values for the oxidation catalytic converter are determined to be above the second characteristic temperature value and if temperature values for the SCR catalytic converter are determined to be below the third characteristic temperature value.
Advantageous embodiments of the invention are illustrated in the drawings and are described below. The features named above explained further below can be used not only in the specified combinations of features, but also in any other combination or alone, without departing from the scope of the present invention.